Paramagnetic oxygen sensing apparatus and method

ABSTRACT

A paramagnetic oxygen sensor and method employs a pressure sensor having a membrane extending through an air gap for a magnetic field. A piezoelectric element is mounted on the membrane. Gas chambers are formed on either side of the membrane. The gas mixture, the properties of which are to be measured, is supplied to one of the chambers. A reference gas is applied to the other chamber. A pulsating magnetic field is provided across the air gap and through the chambers containing the gas mixture and reference gas. The differing responses of the gas mixture and reference gas to the magnetic field deflect the membrane. The deflection of the membrane is sensed by the piezoelectric element. The piezoelectric element maybe operated either in a passive mode or active mode to sense the deflection of the membrane.

CROSS REFERENCE TO RELATED APPLICATION

The present application is the U.S. national stage application ofInternational Application PCT/IB03/01052, filed Mar. 14, 2003, whichinternational application was published on Oct. 2, 2003 as InternationalPublication WO 03/081225. The International Application claims priorityof U.S. Provisional Application No. 60/366,876, filed Mar. 22, 2002.

BACKGROUND OF THE INVENTION

A paramagnetic oxygen sensor employs the following principles. An atomconsists of a nucleus that is surrounded by orbiting electrons. An orbitcan be occupied by up to two electrons, and one or more orbits make upan electron shell. In addition, each electron spins around its own axisand has a magnetic moment associated with the electron spin. Themagnetic properties of the whole atom are then determined by thecombined effect of the spins of all the electrons. Two paired electronsin the same orbit have opposite spins, which cancel their magneticeffects. However, oxygen is one of the rare molecules that has unpairedorbiting electrons around the nucleus and thus a magnetic property. Ithas an even number of electrons orbiting around the nucleus, but two ofthem are in unpaired orbits. This makes the oxygen molecule stronglysusceptible to interaction with an external magnetic field.

The strength of the interaction between a molecule and a magnetic fieldis called magnetic susceptibility. Substances having positive magneticsusceptibility are called paramagnetic and those with negativesusceptibility, diamagnetic. Positive susceptibility means that amolecule is attracted by a magnetic field, negative susceptibility meansthat it is repelled by it. Oxygen is the only gas that is paramagnetic,whereas other gases are weakly diamagnetic. This physical phenomenonoffers a specific way to measure the oxygen content of a respiratory gasmixture, even when nitrous oxide is present.

In 1968 H. Hummel presented a way of using the paramagnetic principle byconstructing a cell in which two gases were mixed inside a homogenousmagnetic field. By using an alternating magnetic field, it is possibleto measure a difference in pressure between the gases in two conduitsupstream of the cell. The amplitude of this signal is directlyproportional to the difference in oxygen partial pressure between thetwo gases to be measured. See U.S. Pat. No. 3,584,499. When the activevolume of the measuring cell is made very small, the response time isfast enough for breath-by-breath measurements. The cell wascommercialized, but it was bulky in size and sensitive to externalvibrations and pressure.

The Datex Division of the Instrumentarium Corporation studied andfurther developed the Hummel cell configuration into a compact, fast,differential cell for measuring oxygen consumption. This oxygen analyzeris described in U.S. Pat. No. 4,633,705. The analyzer, which basicconfiguration is shown in FIG. 1, is constructed of an electromagnetwith a thin air gap ensuring an essentially constant magnetic fieldbetween its poles. The gas to be measured and a reference gas areconducted to this air gap where they are mixed in the uniform magneticfield and then the mixture is conducted out from the gap. A referencegas is needed to measure the absolute oxygen fraction of the measuredgas. A pressure difference proportional to the content of oxygen in thegas exists inside the three conduits entering the gap with a fixedmagnetic field. If the oxygen content in the two gases differs, apressure difference will exist between the inlet conduits outside thegap when the magnetic field is on. By selecting a proper switchingfrequency, the generated pressure signal can be detected by adifferential pressure transducer connected between the inlet conduits.Although this oxygen analyzer is very compact, fast, and accurate itstill has a few disadvantages.

The pressure signal is not measured in the exact spot where it isgenerated. The signal is transferred to the differential pressure sensorvia tubing, connectors and additional volume, which moderate the signalamplitude. The associated pressure transfer function also depends on theproperties of the gas and results in asymmetry between the two branchesof the differential pressure sensor. Interfering mechanical backgroundsignals cause common-mode error, the magnitude of which is affected bythe pressure sensor and gas composition. Some errors are introduced whenthe velocity of the gas changes, as occurs with changes either of thepump power or gas viscosity. The end of the reference tubing is usuallyunder ambient pressure, but the sampling tubing is connected to abreathing circuit. The external pressure disturbance, the overpressuregenerated by the ventilator, is transmitted into cell and is detected bythe pressure sensor. Another type of disadvantage is the continuous needof reference gas flow, which is a disadvantage in closed-circuitanesthesia when room air cannot be used as a reference, because it wouldresult in a slow accumulation of nitrogen in the breathing circuit.

SUMMARY OF THE PRESENT INVENTION

This invention also utilizes the magnetic susceptibility of oxygenmolecules for measuring the oxygen content of a respiratory gas mixture.The pressure difference Δp of oxygen content is measured in the exactspot where it is generated, inside the air gap of an electromagneticcircuit. The magnetic field is switched on and off cyclically. A thin,disk shaped pressure sensor, which is used to measure the partialpressure of oxygen, is located between the magnet poles in the middle ofthe air gap.

The functioning of the pressure sensor is based on piezoelectricity,since piezoelectric crystals can convert mechanical energy to electricalenergy with a good efficiency. Piezoelectric crystals can sensepressures directed to them both passively and actively. In passivesensing the piezoelectric crystal converts the pressure change appliedto it into an electrical voltage spike between its electrodes. Theamplitude of voltage is proportional to intensity of the appliedpressure. Thus in passive sensing, piezoelectric crystal is sensitivefor alternating pressure, but insensitive for static pressure. In activesensing the piezoelectric crystal is excited to a mechanical vibrationnear or at the mechanical resonant frequency of the piezoelectric sensorby applying an alternating electrical voltage to its electrodes.Pressure directed to the vibrating sensor functions as a mechanical loadfor the piezoelectric crystal and can be measured as a change in theelectrical properties of the piezoelectric sensor. Active sensingconsumes energy but it is 10–100 times more sensitive than passivesensing, depending on the Q-value of the transducer.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a view showing a prior art paramagnetic oxygen sensor.

FIGS. 2 a and 2 b show an embodiment of the oxygen sensor of the presentinvention, FIG. 2 b being a cross-sectional view taken along the lineA—A of FIG. 2 a.

FIGS. 3 a and 3 b are a top view and cross-sectional view, respectivelyof one embodiment of a pressure sensor incorporated in the paramagneticoxygen sensing apparatus of the present invention.

FIGS. 4 a and 4 b are a top view and cross-sectional view, respectively,of another embodiment of a pressure sensor for the paramagnetic oxygensensing apparatus.

FIGS. 5 and 6 are cross-sectional views of a portion of the paramagneticoxygen sensing apparatus of FIG. 2 illustrating the operation of thepressure sensor.

FIG. 7 a is an equivalent electrical circuit diagram of a pressuresensor incorporating a piezoelectric element.

FIG. 7 b graphically illustrates a resonance curve of a piezoelectricelement.

FIG. 7 c graphically illustrates a resonance curve for a sensorincorporating a piezoelectric element.

FIGS. 8 a, 8 b, and 8 c are schematic diagrams showing three differenttechniques for measuring the electrical properties of a piezoelectricelement to produce data of the type shown in FIGS. 7 b and 7 c.

FIGS. 9 a, 9 b, and 9 c are graphs illustrating electrical properties ofthe pressure sensor of FIG. 7 a that can be used to measure thedifferential pressures in the paramagnetic oxygen sensing apparatus.

FIG. 10 is a schematic diagram of circuitry for use with a furthertechnique for measuring differential pressures in paramagnetic oxygensensing apparatus.

FIGS. 11 a and 11 b show another embodiment of the oxygen sensor of thepresent invention, FIG. 11 b being a cross-sectional view taken alongthe line of A—A of FIG. 11 a.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

FIG. 2 shows an embodiment of the oxygen sensing apparatus, which has athin, disk shaped piezoelectric pressure sensor mounted inside the airgap of an electromagnetic circuit. The electromagnetic circuit has a potcore type body, which divides into upper half 1 and lower half 2. Thecore is preferably constructed of laminated metal sheets to achieve astrong magnetic field, but iron powders and ferrites may be appropriateas well, when higher frequencies are used. The air gap is formed betweentwo opposite surfaces of upper half center tap 3 and lower half centertap 4. The width of the air gap is approximately 200 μm. The pot corealso encloses coil 5, which is used to produce magnetic flux into theelectromagnetic circuit. The coil is wound around the center tap oflower half of the pot core.

The pressure-sensor 6, which is located in the middle of the air gap,divides the air gap into two different sides in a planar direction. Bothsides form airtight chambers as the pressure sensor is enclosed inside aplastic housing, which divides into cover 7 and body 8. The housing hasgaskets 9 and 10, made of silicon plastic or rubber, which seal up thejoints between the pressure sensors and the pot core taps. The lowerchamber, in FIG. 2, has an inlet 11 to conduct a respiratory gas mixturethrough cavity 12 to between the lower surface of pressure sensor andthe upper surface of lower pot core tap. The lower chamber also has anoutlet 13, to which a gas pump is connected. The outlet is connected tothe lower chamber through a cavity 14. The pump is used to create anunderpressure to the outlet, which causes a gas flow through the chamberfrom the inlet to the outlet. The gas inlet and outlet, as well aselectrical connections for the coil and pressure sensor are brought outfrom the oxygen sensing apparatus through the junction point of theupper and lower halves of the pot core.

The upper chamber has cavities 15 connected to ambient atmosphericpressure through cavities 16 in upper half 1 of the pot core to maintaina constant pressure inside the chamber. No gas flow is needed for thereference side. The pressure sensor is insensitive for constantpressures and a constant underpressure on the sample side produced bythe pump cause a zero voltage at the sensor output. However, asinusoidal pressure ripple caused by the pumping action of the pump'spiston, can still be seen as a corresponding sinusoidal voltage at thesensor output. The frequency of this disturbance can be filtered outfrom the output signal since it is much lower compared to switchingfrequency 0.5–5 kHz of the pulsating magnetic field.

FIG. 3 b shows a cross sectional view of an embodiment for the pressuresensor 6. The sensor is constructed of a ring shaped piezoelectriccrystal 20, which is attached on the top of a circular membrane 21. Theattachment is electrically conductive and is done by gluing, solderingor other similar technique. The piezoelectric element can be one layeror a multilayer bi-morph. The thickness of the piezoelectric element isbetween 25–500 μm, outer diameter between 10–20 mm, and inner diameterbetween 2–4 mm. The circular membrane is preferably made of anonmagnetic material such as BeCu or spring steel, but other materialsmay fit as well. The thickness of the membrane is between 5–50 μm anddiameter between 10–20 mm. The membrane is electrically connected to alower electrode of the piezoelectric crystal and functions as a groundelectrode for the crystal. An electric wire is soldered to anotherelectrode, on the upper surface of the piezoelectric crystal and isconnected to measuring electronics. Pressure applied to the membraneside of the sensor structure bows the membrane, which then bends thepiezoelectric crystal. This bending causes the electrical properties ofpiezoelectric crystal to change and a voltage to appear between theelectrodes of the piezoelectric crystal. This type of passive sensor cansense very low pressures between 0–5 Pa with the accuracy error of lessthan 5 mPa. The thin construction of the sensor also enables it to beplaced into the narrow cavity between the pot core taps of theelectromagnetic circuit.

FIG. 4 b shows a cross sectional view of another embodiment for thepressure sensor element. The sensor is constructed of a ring shapedpiezoelectric crystal 23 attached to the junction point of a circularmembrane 24 and a ring shaped supporting structure 25. The attachment iselectrically conductive and is done by gluing, soldering or othersimilar technique. The piezoelectric element can be one layer or amultilayer bi-morph. The thickness of the piezoelectric element isbetween 25–500 μm, outer diameter between 10–20 mm, and inner diameterbetween 2–4 mm. The circular membrane is preferably made of nonmagneticmaterial, such as BeCu or spring steel, but other materials may fit aswell. The thickness of the membrane is between 5–50 μm and diameterbetween 10–20 mm. The ring shaped supporting structure is preferablymade of metal. The thickness of the structure is between 100–1000 μm,outer diameter between 10–20 mm, and inner diameter between 10–18 mm.Membrane 24 is electrically connected to a lower electrode ofpiezoelectric crystal 23 and upper surface of supporting structure 25.The supporting structure thus functions as a ground electrode of thecrystal through the membrane. An electric wire is soldered to anotherelectrode, on the upper surface of piezoelectric crystal 23 and isconnected to measuring electronics. Pressure applied to the membraneside of the sensor structure bows the membrane and causes a twistingforce at the hinge point of the membrane and supporting structure. Thistwisting is transferred to the piezoelectric crystal, which causes theelectrical properties of the piezoelectric crystal to change and avoltage to appear between the electrodes of the piezoelectric crystal.

The functioning of the pressure sensor is shown in the cross-sectionalof views in FIGS. 5 and 6. A gas pump connected to gas outlet 13 createsa gas flow, of for example breathing gases, through the lower chamber ofthe housing from inlet 11 to outlet 13. The underpressure created by thepump bows the pressure sensor slightly downwards, but the voltage output17 of the pressure sensor is zero, since the piezoelectric element isinsensitive for constant pressure directed to it. In addition to thepressure sensor bowing, it also vibrates a little in the phase of thesinusoidal piston action of the pump. The frequency of this disturbanceis less than 100 Hz, normally around 50–60 Hz, which can be seen as acorresponding sinusoidal voltage at the output 17. Although theunderpressure inside the lower chamber does not have much effect on theoutput voltage of the pressure sensor it should be minimized to preventthe sensor membrane from touching the center tap of the pot core.Minimization can be done with pneumatic designing by keeping thepressure difference between the gas inlet and outlet as small aspossible. The sinusoidal pressure ripple caused by the pump is minimizedwith pneumatic filtering. Furthermore the voltage signal from the sensoroutput is electrically filtered with band pass filtering, since the pumpfrequency is much lower than the frequency of output signal.

Now, when the magnetic field is switched on and off cyclically, analternating pressure signal proportional to the content of oxygen in themeasured gas is generated. When the switching cycle is at off state, themagnetic field in the air gap is zero as shown in FIG. 5. The pressuredifference between the reference gas in the upper chamber and sample gasin the lower chamber is constant and electrical output of the pressuresensor is zero. When the switching cycle of the pulsating magnetic fieldis at state of high field strength, as shown in FIG. 6, oxygen moleculesin the reference and sample gas move from outside the air gap towardsthe gradient of the magnetic field appearing near the edges around theair gap. When the partial pressures of oxygen are equal on the referenceand sample sides, the pressure difference created during the on state ofthe magnetic field is zero. This is the case when the partial pressureof oxygen on the sample side equals that of ambient air, that isapproximately 21% at 101.325 kPa. As the partial pressure of oxygenincreases on the sample side, while the partial pressure of oxygen staysconstant on the reference side, the pressure sensor bows during the onstate of the pulsating magnetic field, as shown in FIG. 6. The voltageappearing at the electrodes of pressure sensor 6 is proportional to theamplitude of the bowing, and thus to the pressure difference between thereference side and the sample side. The frequency of the pressure signalcorresponds to the switching frequency of the pulsating magnetic field,which may be between 0–100 kHz, but preferably 0.1–5 kHz. The maximumpressure difference between 100% oxygen on the sample side and 21% onthe reference side can be calculated from the equation below to beapproximately 3 Pa, with the magnetic field strength B=2.4 T andtemperature T=318.15 K.

The force acting on a molecule in a magnetic field is equal to themagnetic susceptibility of the molecule multiplied by the product of themagnetic field strength and its gradient. In a constant magnetic fieldthe gradient is zero. The molecule can experience a force only in areaswhere the magnetic field is gradually changing. In a practical magneticcircuit, a constant magnetic field exists inside an air gap between themagnet poles. Outside the gap, the field is rapidly falling to zero. Thepressure difference is thus generated between the gas inside and outsidethe magnetic field. The pressure difference Δp between the high magneticfield area and zero field area is proportional to the partial pressureof oxygen pO₂ in a gas mixture:

$\begin{matrix}{{{\Delta\; p} = {\left( \frac{C_{m}}{2 \cdot \mu_{0} \cdot R} \right) \cdot {{pO}_{2}\left( \frac{B}{T} \right)}^{2}}},{where}} & (1)\end{matrix}$

-   C_(m)=1.36·10⁻⁵ K/mol is Curie constant per mol-   μ₀=4·10⁻⁷ Vs/Am is vacuum permeability-   R=8.3144 J/Kmol is gas constant-   B[T] is magnetic field strength-   T [K] is temperature

As the frequency for measuring the oxygen content is fixed to a certainpre-determined value and as most of the disturbances are found fromcertain bandwidths, it is possible to do some pressure signalpre-filtering already at the pressure sensor. When the piezoelectricelement is attached to another mechanical element, for example membrane21 as shown in FIG. 3, to form a pressure sensor, the attachment affectsthe mechanical resonance frequency of the piezoelectric element. Theattached element functions as the mechanical load to the piezoelectricelement. The attached mechanical element will not necessarily vibrate atthe same frequencies as the piezoelectric element so that the overallcomposite sensor construction may have a plurality of mechanicallyresonant frequencies and a plurality of anti-resonant frequencies. Theoutput signal of the pressure sensor is much weaker at anti-resonantfrequencies than at resonant frequencies. The frequencies of mechanicalresonance of a piezoelectric element is established by the externaldimensions of the element and/or the composition of the piezoelectricmaterial forming the element and can be changed by changing theseaspects of the element. This means that the pressure sensor can be soformed as to be mechanically “tuned” to a local resonant frequency,corresponding the frequency of oxygen content measurement, to get ahigher output signal at that bandwidth, but a lower output outside ofthat bandwidth.

The foregoing description describes operation of the paramagnetic oxygensensing apparatus in a “passive” mode. The following descriptiondescribes operation of the sensing apparatus in an “active” mode.

When the pressure sensor is used actively, it is excited to vibrationnear or at the mechanical resonant frequency of the sensor by applyingan alternating electrical voltage to its electrodes. An external force,such as the pressure inside the lower chamber, directed to the vibratingsensor functions as a mechanical load for the piezoelectric crystalelement and can be measured as a change in electrical properties of theelement. Active sensing consumes energy, but it is more sensitive thanpassive sensing. The sensitiveness depends on the Q-value of the sensor,which determines the sharpness and the amplitude of the resonantfrequency spike. On the other hand, the bandwidth of the sensor becomesnarrower as the sensitiveness is increased. The main difference betweenactive and passive sensing is that the pressure sensor can also sensestatic pressures in active sensing. This means that the sensor issensitive to slow pressure changes such as pressure changes in breathingcircuit or static underpressure of the pump. These errors can beminimized with pneumatic circuit design and signal filtering.

The pressure sensor shown in FIG. 2 is also suitable for the use inactive sensing on a condition that static pressures are minimized withpneumatic design to such level that the signal detection stays in alinear detection region.

Piezoelectric element 20 of FIGS. 2, 3, and 4 can be used toelectrically measure the mechanical strains in the piezoelectric elementcaused by the external forces applied to the element. These pressuresresult from differential gas pressures between the chambers acting onmembrane 21 in the manner described above in connection with FIGS. 5 and6. The effects of these mechanical-electrical conversions are mostpronounced when they occur along the poling axis of the piezoelectricelement established during manufacture and piezoelectric element isformed and mounted to plate 21 such that this will occur. The polingaxis will be generally parallel to the plane of a ring-likepiezoelectric element 20.

When a mechanical force or electrical energization is applied to apiezoelectric element that does not thereafter change or changes onlyvery slowly, the conversion occurring in the piezoelectric element issomewhat ineffective. For example, the dimensional change in apiezoelectric element resulting from the application of a DC electricalenergization is usually measured in nanometers. The conversion frommechanical energy to electrical energy is somewhat more effective andbecomes more effective if the mechanical force is rapidly applied. Forexample, delivering a sharp blow to a piezoelectric element results inan output voltage spike.

When electrical energization that alternates in polarity is applied to apiezoelectric element, the piezoelectric element undergoes mechanicalvibration at a frequency corresponding to that of the alternatingelectrical energization. A piezoelectric element, like other themechanical objects and structures, will have a natural frequency ofvibration. When vibrating at the natural frequency, physicaldisplacements in an object are at maximum amplitude. When the frequencyof the alternating electrical energization is that of the naturalfrequency of the piezoelectric element, the condition is one ofmechanical resonance. When converting electrical energy into mechanicalenergy at the frequency of mechanical resonance of the piezoelectricelement, the maximum amplitude of mechanical displacement induced in thepiezoelectric element by the alternating electrical energization is muchgreater, for example, 10–100 times greater, than the maximumdisplacement that can be obtained from the application of electricalenergization that does not change or changes very slowly afterapplication.

To consider the conversion of mechanical energy to electrical energywhen a piezoelectric element is driven at the frequency of mechanicalresonance, the simple equivalent circuit shown in FIG. 7 a mayillustrate the electrical characteristics of the piezoelectric element.In the equivalent circuit, capacitance Co is the capacitance of thepiezoelectric element and resistance Ro is the dielectric loss of thepiezoelectric element. Resistor R1 represents the mechanical loss in thepiezoelectric element and resistance Rm represents the mechanical loadon the sensor, such as that imposed by membrane 21. Capacitor C1 andinductor L1 represent the rigidity and mass of the material of thepiezoelectric element, respectively.

The series and parallel connections of the capacitive and inductivecomponents in the equivalent circuit shown in FIG. 7 a cause the overallcircuit impedance characteristics to vary with frequency. When apiezoelectric element is vibrated at the frequency of mechanicalresonance, the impedance of the piezoelectric element is at its lowestvalue. The inverse expression of impedance is “admittance,” whichquantity is used herein for ease of explanation. The admittance ofpiezoelectric element will be at its greatest value at the frequency ofmechanical resonance of the piezoelectric element. Conditions at thisfrequency resemble the characteristics of a series connected,inductive-capacitance alternating current circuit and are sometimescalled that of electrical “resonance.”

In addition to the high admittance characteristics appearing at thefrequency of mechanical resonance, there will also be a vibrationfrequency at which the admittance of the piezoelectric element will beat a minimum value. Conditions at this frequency resemble those of aparallel inductive-capacitance alternating current circuit and thispoint is sometimes called that of “anti-resonance.”

In FIG. 7 b, the ordinate is scaled in the electrical admittance Y ofthe piezoelectric element. The abscissa is scaled in the frequency. Thegraph 200 of FIG. 7 b shows the electrical admittance Y of apiezoelectric element with respect to the mechanical resonance frequencyof the piezoelectric element. The frequency 202 at which the admittanceY is at a maximum value is the mechanical resonance frequency of theelement. The minimum value of admittance Y is found at frequency 204,which is characterized as the anti-resonance frequency.

As noted above, the frequency of mechanical resonance of a piezoelectricelement is established by the external dimensions of the element and/orthe composition of the piezoelectric material forming the element andcan be changed by changing these aspects of the element.

When the piezoelectric element is attached to another mechanicalelement, for example, membrane 21, as shown in FIG. 3, to form apressure sensor, the attachment affects the mechanical resonancefrequency of the piezoelectric element. The attached element functionsas the mechanical load to the piezoelectric element. The attachedmechanical element will not necessarily vibrate at the same frequenciesas the piezoelectric element, so that the overall composite sensorconstruction may have a plurality of mechanically resonant frequencies,at which the admittance Y is at high values, and a plurality ofanti-resonant frequencies, at which the admittance has low values.

FIG. 7 c shows, in a manner similar to FIG. 7 b, a graph 206 ofadmittance Y versus frequency for a composite structure, such as thatdescribed above. Vibration of the composite structure at frequencies208, 210 and 212 produce high values of admittance. Vibration atfrequencies 214 and 216 produce low values for admittance Y.

As generally indicated in FIGS. 7 b and 7 c, the admittance Y values atthe peaks of the resonance frequencies are from several to 1000 timeshigher than those values found in the lower portions of the graph. Thewidth of the peaking portions of the admittance-frequency graphs, interms of frequency at −3 dB admittance level, is usually from tens ofhertz to several kilohertz, depending on the structure of thepiezoelectric element and/or composite structure.

The graphs shown in FIG. 7 may be obtained by measuring the currentthrough a piezoelectric element against the frequency of the electricalsignal applied to the piezoelectric element when an alternatingelectrical energization of constant peak voltage magnitude is applied tothe piezoelectric element. The measured current is used to compute theadmittance of the piezoelectric element. The frequency that produces thehighest current, and hence highest admittance, is the mechanicalresonance frequency of the element. FIG. 8 a shows circuit that may beused to determine the admittance of a composite construction containinga piezoelectric element. Piezoelectric element 20 is connected in serieswith ammeter 220 across constant peak voltage magnitude, variablefrequency AC voltage source 222. As the frequency of voltage source 222is varied, the current through piezoelectric element 20 is measured andthe admittance determined as Y=I/V.

Or a current that alternates between fixed magnitudes may be applied tothe piezoelectric element as shown in FIG. 8 b. The current source 224is of adjustable frequency. The voltage across the piezoelectric elementis measured by voltmeter 226 as the frequency of the applied current isvaried. With the current magnitude so fixed, the voltage across thepiezoelectric element will decrease as the admittance of thepiezoelectric element increases at the frequency of mechanicalresonance. The same formula, Y=I/V, is used to determine admittance.

A third way to establish the data shown in FIG. 7 is to measureelectrical phase differences occurring in the circuit containingpiezoelectric element 20. At the frequency of mechanical resonance,there will be a minimum phase difference, or no phase difference,between the voltage and current in the circuit. See FIG. 8 c in whichthe phase difference may be determined by voltage and load currentmeasurements carried out in connection with resistor 228 and voltagesource 230.

An external compressive or tensile load applied to the vibratingpiezoelectric structure, as when membrane 21 is subjected to overpressure in the lower chamber, shifts the series resonance frequency orfrequencies, such as 202, 208, 210 and 212, and the parallel oranti-resonant frequency or frequencies, such as 204, 214, and 216. Theshift in resonance and anti-resonance frequencies will be related to themagnitude of the applied load. Furthermore, the shift in resonance andanti-resonance frequencies for a given applied load is greater when theeffect of external force is directed along the poling axis of thepiezoelectric element. The characteristics described above are used tomeasure differential pressures of oxygen content in a breathing gasbetween the upper and the lower chambers of the pressure sensor inpresence of pulsating magnetic field in the following manner.

For explanatory purposes, FIG. 9 a shows a simple admittance-frequencycurve 300, similar to that shown in FIG. 7 b. It will be appreciatedthat the actual admittance-frequency curve for a sensor will moregenerally resemble that of FIG. 7 c since piezoelectric element 20 iscoupled to membrane 21 to form a composite pressure sensor structure.Piezoelectric element 20 is energized at resonance frequency 202.

As shown in connection with FIGS. 5 and 6, pressure will be applied tomembrane 21 of pressure sensor 6 by the partial pressure difference ofoxygen between the two chambers. These pressures will, in turn, beapplied to piezoelectric element 20. The mechanical loading applied topiezoelectric element 20 will cause the resonance frequency of thepiezoelectric pressure sensor structure to shift from frequency 202 tofrequency 242, as shown on in FIG. 9 a by graph 244. The direction ofthe shift will depend on the construction of the piezoelectric pressuresensor structure and on whether the mechanical load applied topiezoelectric element 20 is tensile or compressive. With the resonancefrequency curve shifted to that shown by graph 244, the admittance Y ofthe pressure sensor structure measured at the energization frequency 202will fall to the level 246. The difference in admittance between level240 and level 246 is a measure of the differential gas pressure betweenthe two chambers. The peaking nature of the graph shown in FIG. 4 a atthe resonance frequency is useful in providing difference values of amagnitude that assists in accurately determining pressures.

FIG. 9 b shows the situation in which the pressure applied to membrane21 by the gases in the two chambers results in a loading ofpiezoelectric element 20 that causes the resonance frequency toincrease, as shown in the figure by frequency 248 and curve 250. Theadmittance value Y measured at frequency 202 falls to a level 252 lowerthan level 240 that may be used to determine partial pressure of oxygenin breathing gas.

While FIGS. 9 a and 9 b have described operation of pressure sensorusing resonance frequency 202, it will be appreciated that thedifferential gas pressure measuring technique described above will alsowork should pressure sensor be operated at a frequency other than theresonance frequency. The difference in admittance Y values between theunloaded and loaded states of the piezoelectric sensor structure willtend to be less than those obtained through the use of the resonancefrequency 202 and shown in FIGS. 9 a and 9 b.

This technique to measure gas pressure magnitude is shown in FIG. 9 c.In this technique, the characteristics of admittance versus frequency,shown graphically in FIG. 9 c as curve 260, are determined for a statein which the piezoelectric pressure sensor structure is not subject toany mechanical loading. The graph will exhibit a resonance frequency262.

Pressure sensor 6 is then operated to supply electrical energization topiezoelectric element 20 at a frequency 264, different from frequency262 and the admittance Y of the unloaded state is measured, as level 266which value is used as a reference signal.

Thereafter, the piezoelectric pressure sensor structure is subjected tothe differential pressure caused by the gases oxygen content differencebetween the two chambers in presence of a pulsating magnetic field. Themechanical loading applied to piezoelectric element 20 by thedifferential pressure will shift the admittance-frequency curve, asshown in FIG. 9 c by graph 268. This shift will cause the admittance ofthe piezoelectric pressure sensor structure measured at frequency 264 tochange to the value indicated by level 272. The change in admittancevalue can be used to determine the oxygen content of the breathing gas.

The frequency 264 used for measuring purposes can be chosen inaccordance with the construction of the piezoelectric pressure sensorstructure and the minimum and maximum differential pressures to bemeasured. It is usually spaced tens or hundreds of hertz greater orlower than the resonance frequency 262. Also, it is desirable to selecta frequency 264 that lies in a generally linear portion of graph 260 forthe range of differential pressures to be measured. This provideslinearity in the measurement of differential pressure within thepressure range. A linear portion of curve 260 is shown by line 274 anddots 276 a and 276 b.

When the mechanical loading applied to piezoelectric element 20 by thedifferential pressure on membrane 21 is opposite to that describedabove, the admittance versus frequency curve will shift in the oppositedirection from that described above. This is shown by the partial curve278 in FIG. 9 c. In this circumstance, the admittance value Y measuredat frequency 264 will decrease to level 280. The difference between theadmittance value 266 and the admittance value 280 may be used todetermine the oxygen content of the breathing gas. The fact that theadmittance value 280 is decreased from admittance value 266 indicatesthat the loading on piezoelectric element 20 is opposite that whichproduces admittance level 272.

While FIG. 9 c shows operation of pressure sensor at a frequency 264less than resonance frequency 262, it will be appreciated that pressuresensor may be operated in an analogous manner for a frequency greaterthan frequency 262. The changes in admittance caused by a compressiveloading of piezoelectric element 20 and a tensile loading of thepiezoelectric element will be opposite to that described above inconnection with FIG. 9 c.

A benefit achieved in measuring the partial pressure of oxygen at afrequency point aside from the natural resonant frequency point is lowerpower consumption. However, to ensure that the admittance measurementsare sufficient to measure pressure changes with the desired degree ofaccuracy, the amplitude of alternating voltage supplied to thepiezoelectric element 20 must be sufficiently high to provide thedesired signal to noise the ratio in the signals used for measurement.

A further technique to measure differential pressure magnitude betweenthe two chambers is based on electrical phase differences between thevoltage and current in piezoelectric sensor 6 and is shown in FIG. 10. Acircuit for measuring phase difference includes resistor 50 in serieswith piezoelectric element 20. Resistor 50 corresponds to resistor 228shown in FIG. 8 c. The voltage across resistor 50 is an indication ofthe current through piezoelectric element 20. The output of amplifier 51containing the amplified output of voltage controlled oscillator 52 isan indication of the voltage applied to piezoelectric element 20. Thecurrent signal from resistor 50 and the voltage signal from theamplifier 51 are applied to phase displacement detection system 53respectively. Phase displacement detection system 53 determines thephase difference between the two signals, as by detecting zero crossingsor some other appropriate technique, and provides a phase differenceoutput signal.

At the resonance frequency of the pressure sensor, when there is nomechanical loading on the sensor due to a zero pressure or “normal”conditions inside the two chambers of pressure sensor 6, the phasedifference between the current and voltage is zero or close to zero.When a pressure difference occurs between the two chambers, as when themagnetic field is connected on, this shifts the resonance frequency ofthe pressure sensor, for example, to a lower frequency than thefrequency of the output signal of voltage controlled oscillator which isat the resonance frequency of the piezoelectric sensor in the zero gaspressure difference state. These conditions result in a phase differencebetween the current as reflected in the voltage measured across resistor50 and the voltage output of amplifier 51. The phase difference may beone in which the phase of the current is behind the phase of thevoltage. If the voltage signal is used as a reference, the phasedifference may be deemed a “negative” phase difference; i.e. the currentlags the voltage.

This “negative” phase difference is detected by phase displacementdetection system 53. Phase displacement detection system 53 thencontrols voltage-controlled oscillator 52 by, for example, decreasingthe oscillator control voltage to alter the frequency of the voltagecontrolled oscillator to minimize the phase difference. As theoscillator control voltage is decreased, the oscillator output frequencyalso decreases and the phase difference between the current and thevoltage decreases. As the magnitude of pressure difference ceasesbetween the chambers, the need to decrease the oscillator controlvoltage also lessens. Finally, when the differential pressure betweenthe chambers has reached its minimum value, the phase difference againbecomes zero, due to the fact that the energization frequency fromvoltage controlled oscillator 52 has been set to the resonance frequencyof the pressure sensor at the minimum pressure difference condition. Atthis point, the oscillator control voltage from phase displacementdetection system 53 is minimum. The change in oscillator control voltageprovided by phase displacement detection system 53 is an indication ofthe partial pressure difference of oxygen between the two chambers ofthe pressure sensor in presence of a pulsating magnetic field.

As the pressure difference between the two chambers of pressure sensor 6starts to revert back to its original condition, as the magnetic fieldis removed, the phase difference between the current and the voltageagain increases but in the opposite direction, i.e. a “positive” phasedifference. The new resonance frequency point which was establishedshifts back to the original resonance frequency as the differentialpressure returns to the zero pressure or baseline condition. The“positive” phase difference is detected by phase displacement detectionsystem 53, which then increases the oscillator control voltage forvoltage controlled oscillator 52 toward its original value to againminimize the phase difference between the current and voltage inpressure sensor 6. As the oscillator control voltage is increased, theoscillator output frequency also increases and the phase difference inpressure sensor 6 decreases. As the differential pressure between thetwo chambers of pressure sensor 6 reaches its original value, the phasedifference becomes minimized, or zeroed, as the oscillator controlvoltage and oscillator output frequency reach the same values thatestablished the original zero phase difference.

If desired, it is possible to create a reference gas flow through theupper chamber similar to gas flow through the lower chamber. Anotherembodiment of mechanical construction for the pressure sensor is shownin FIG. 11. The construction in FIG. 11 is identical to that shown inFIG. 2 with a difference that reference gas flow through the upperchamber is similar to gas flow through the lower chamber, which is tominimize static pressure between the two chambers. The reference gasflow is conducted into the upper chamber through reference gas inlet 17and conducted out through reference gas outlet 18. Reference gas outlet18 and sample gas outlet 13 are connected through identical pneumaticcircuits to a pressure pump. Reference inlet 17 and sample inlet 11 aresimilarly connected to identical pneumatic circuits with a differencethat sample inlet may be connected to a patient breathing circuit.Pressures inside both chambers of the pressure sensor should be equal tokeep the sensor signal detection in a linear detection region.

1. A sensing apparatus for measuring the amount of a given gas in a gasmixture, said sensing apparatus utilizing the magnetic susceptibilityproperties of the given gas and comprising: a magnetic core having apair of elements spaced to form an air gap; means for generating amagnetic field that traverses said air gap; a deflectable membraneextending through said air gap; means forming a chamber on each side ofsaid membrane; means for supplying the gas mixture to one of saidchambers, the other of said chambers containing a reference gas; and amechanical-electrical conversion element mounted on said membrane andresponsive to deflection of said membrane by the gas mixture andreference gas in said chambers when a magnetic field is present in theair gap to provide an indication of the amount of the given gas in thegas mixture.
 2. The sensing apparatus according claim 1 wherein saidmechanical-electrical conversion element comprises a piezoelectricelement.
 3. The sensing apparatus according claim 1 wherein saidmechanical-electrical conversion element is formed as an annular membermounted on said membrane.
 4. The sensing apparatus according to claim 2wherein said piezoelectric element is mounted on said membrane so thatthe mechanical-electrical conversion occurring in said piezoelectricelement occurs along a poling axis of said piezoelectric element.
 5. Thesensing apparatus according to claim 1 further including a support formounting said membrane to said chambers.
 6. The sensing apparatusaccording to claim 1 further including means for providing a flow of thegas mixture through said one of said chambers.
 7. The sensing apparatusaccording to claim 1 wherein said other of said chambers containsambient air.
 8. The sensing apparatus according to claim 6 wherein saidother of said chambers contains ambient air.
 9. The sensing apparatusaccording to claim 1 further including means to provide a flow of thereference gas through the other of said chambers.
 10. The sensingapparatus according to claim 1 wherein said mechanical-electricalconversion element generates an electrical signal responsive to thedeflection of said membrane and wherein a magnitude of said electricalsignal provides an indication of the amount of the given gas in the gasmixture.
 11. The sensing apparatus according to claim 1 wherein saidmechanical-electrical conversion element vibrates mechanicallyresponsive to the application of alternating electrical energization tosaid element, said element having an admittance, the admittance of saidelement at a given frequency of alternating electrical energizationbeing alterable by deflection of the membrane, said sensing apparatusfurther comprising; means for applying alternating electricalenergization to said element; and admittance measuring means coupled tosaid element for measuring the admittance exhibited by said element whenalternating electrical energization is applied to said element and saidmembrane is deflected by the gas mixture and reference gas in saidchambers to provide an indication of the amount of the given gas in thegas mixture.
 12. The sensing apparatus according to claim 11 whereinsaid admittance measuring means is further defined as measuring theadmittance exhibited by said element at a resonant frequency alternatingelectrical energization.
 13. The sensing apparatus according to claim 11wherein said admittance measuring means is further defined as measuringthe admittance exhibited by said element at a frequency other than theresonant frequency of alternating electrical energization.
 14. Thesensing apparatus according claim 11 wherein said admittance measuringmeans comprises: means for applying alternating electrical energizationhaving a desired voltage property which is constant in magnitude to saidelement; means for measuring the current through said element; and meansfor determining the admittance exhibited by said element from thevoltage property and the measured current value.
 15. The sensingapparatus according to claim 11 wherein said admittance measuring meanscomprises: means for applying alternating electrical energization havinga desired current property which is constant in magnitude to saidelement; means for measuring the voltage across said element; and meansfor determining the admittance exhibited by said element from thecurrent property and the measured voltage.
 16. The sensing apparatusaccording to claim 11 wherein said admittance measuring means comprisesmeans for ascertaining phase shifts occurring in the alternatingelectrical energization as a result of its application to said element.17. The sensing apparatus according to claim 1 further defined as asensing apparatus for measuring the amount of a given gas in a gasmixture by utilizing the positive magnetic susceptibility, paramagneticproperties of the given gas.
 18. The sensing apparatus according toclaim 17 further defined as a sensing apparatus for measuring the amountof oxygen in a gas mixture.
 19. The sensing apparatus according to claim18 further defined as one for measuring the oxygen content of thebreathing gases of a subject.
 20. A method for measuring the amount of agiven gas in a gas mixture utilizing the magnetic susceptibilityproperties of the given gas, said method comprising the steps of: (a)forming an air gap across which a magnetic field can flow; (b) placing adeflectable membrane through the air gap; (c) forming a gas chamber oneach side of the membrane; (d) supplying the gas mixture to one of saidchambers, the other of said chambers containing a reference gas; (e)periodically providing a magnetic field across the air gap and throughthe chambers containing the gas mixture and reference gas, differingresponses of the gas mixture and the reference gas to the magnetic fielddeflecting the membrane; and (f) sensing the deflection of the membraneas a measurement of the amount of the given gas in the gas mixture. 21.The method according to claim 20 further defined as providing amechanical-electrical conversion element in operative association withthe membrane for sensing the deflection of the membrane and forproviding an indication of the amount of the given gas in the gasmixture.
 22. The method according to claim 21 further defined asproviding a piezoelectric element in operative association with themembrane for sensing the deflection of the membrane.
 23. The methodaccording to claim 22 further defined as sensing the deflection of themembrane by mechanical-electrical conversion occurring along a polingaxis of the piezoelectric element.
 24. The method according to claim 20wherein the step of supplying the gas mixture to one of the chambers isfurther defined as flowing the gas mixture through the one of thechambers.
 25. The method according to claim 20 wherein the other of thechambers contains ambient air.
 26. The method according to claim 24wherein the other of the chambers contains ambient air.
 27. The methodof claim 20 further defined as flowing reference gas through the otherof the chambers.
 28. The method according to claim 20 wherein step (e)is further defined as providing the magnetic field at a switchingfrequency of up to 100 kHz.
 29. The method according to claim 28 whereinstep (e) is further defined as providing the magnetic field at aswitching frequency of 0.1 to 5 kHz.
 30. The method according to claim21 wherein the mechanical-electrical conversion element generates anelectrical signal responsive to the deflection of the membrane, themagnitude of which provides an indication of the amount of the given gasin the gas mixture.
 31. The method according to claim 21 wherein themechanical-electrical conversion element mechanically vibratesresponsive to the application of alternating electrical energization tothe element, the element having an admittance, the admittance of theelement at a given frequency of alternating electrical energizationbeing alterable by a mechanical loading of the element, said methodfurther including the steps of: (g) applying alternating electricalenergization to the element at a selected frequency; (h) measuring theadmittance exhibited by the element when subjected to loading by thedeflection of the membrane and energized by electrical energization ofthe selected frequency; and (i) using the admittance properties of themechanical-electrical conversion element as a measurement of the amountof the given gas in the gas mixture.
 32. The method according to claim31 wherein the mechanical-electrical conversion element has a resonancefrequency at which the admittance of the element has a peak value andwherein step (g) is further defined as applying alternating electricalenergization to the element at the resonance frequency.
 33. The methodaccording to claim 31 wherein the mechanical-electrical conversionelement has a resonance frequency at which the admittance of the elementhas a peak value and wherein the step (g) is further defined as applyingalternating electrical energization to the element at a frequency otherthan the resonance frequency.
 34. The method according to claim 31further including a step (j) of measuring the admittance exhibited bythe element when the membrane is in an unloaded state when energized bythe electrical energization of the selected frequency and wherein themethod further includes the step (k) of measuring the difference betweenthe admittances measured in step (h) and (j) as a measurement of theamount of the given gas in the gas mixture.
 35. The method according toclaim 31 wherein the step of measuring the admittance exhibited by theelement further comprises the steps of: applying alternating electricalenergization having a desired voltage property which is constant inmagnitude to the element; measuring the current through the element; anddetermining the admittance exhibited by the element from the voltageproperty and measured current value.
 36. The method according to claim31 wherein the step of measuring the admittance exhibited by the elementfurther comprises the steps of: applying alternating electricalenergization having a desired current property which is constant inmagnitude to the element; measuring the voltage across the element; anddetermining the admittance exhibited by the element from the currentproperty and the measured voltage.
 37. The method according to claim 31wherein the step of measuring the admittance exhibited by the element isfurther defined as ascertaining phase shifts in the alternatingelectrical energization.
 38. The method according to claim 20 furtherdefined as a method for measuring the amount of a given gas in a gasmixture utilizing the positive magnetic susceptibility, paramagneticproperties of the give gas.
 39. The method according to claim 20 furtherdefined as a method for measuring the amount of oxygen in a gas mixture.40. The method according to claim 39 further defined as a method formeasuring the oxygen content of the breathing gases of a subject.